[1] The declension in the singular should be nominative μήτηρ with lengthened e-grade, vocative μῆτερ , genitive and dative oxytone with zero-grade of the stem μητρός and μητρί , accusative paroxytone with e-grade of the stem μητέρα . But in epic/poetic language the genitive and dative can show the e-grade, which is probably a secondary development by analogy and/or metri causa and not, as implied in this entry, an original stage from which the more common form develops by syncope; cf. Etymologicum Gudianum 394.6, Etymologicum Magnum 584.53, and LSJ s.v. μήτηρ (web address 1).
[2] In the dative plural, -ρα- comes from an inherited vocalic r. The paroxytone accent has been attributed (not without dispute) to Wheeler's Law, on the assumption that the original accentuation was oxytone. See bibliography below.